Integrated circuit dies are conventionally enclosed in plastic packages that provide protection from hostile environments and enable electrical interconnection between the integrated circuit die and an underlying substrate such as a printed circuit board (PCB). The leadframe is the central supporting structure of such a package. A portion of the leadframe is internal to the package, i.e., completely surrounded by the plastic encapsulant.
For purposes of high-volume, low-cost production of chip packages, a current industry practice is to etch or stamp a thin sheet of metal material to form a panel or strip that defines multiple leadframes. A single strip may be formed to include multiple arrays, with each such array including a multiplicity of leadframes in a particular pattern. In a typical semiconductor package manufacturing process, the integrated circuit dies are mounted and wire bonded to respective ones of the leadframes, with the encapsulant material then being applied to the strips to encapsulate the integrated circuit dies, bond wires, and portions of each of the leadframes in the above-described manner.
Upon the hardening of the encapsulant material, the leadframes within the strip are cut apart or singulated for purposes of producing the individual semiconductor packages. Such singulation is typically accomplished via a saw singulation process. In this process, a saw blade is advanced along “saw streets” which extend in prescribed patterns between the leadframes as required to facilitate the separation of the leadframes from each other in the required manner.
In current, conventional leadframe design, the leadframe does not define a continuous, uninterrupted surface. Rather, individual leads of the leadframe are separated from each other and from the peripheral edge of a die pad (if included in the leadframe) by narrow gaps. The die pad of the leadframe, if included therein, is the supporting structure to which the die is typically attached.
In conventional leadless semiconductor packages, an adhesive tape is attached to the bottom of the leadframe to provide mechanical support and rigidity for the leadframe structure during material handling in the assembly process. The adhesive tape also helps prevent mold flash during the molding process. However, the adhesive tape contributes to the bouncing lead effect during the wire bonding process, which may result in poor wire bond quality, and/or non-stick on lead (NSOL) problems. The adhesive tape also may hinder stabilization of half-etched lead fingers during wire bonding.
In flip chip leadless semiconductor packages, die bond pads connected from solder bumps through half-etched lead fingers to the external leads of the semiconductor package. A support block typically is used to stabilize the lead fingers; however, the support block can become obstructed with the use of adhesive tape.
Film assisted molding equipment has been developed to address these problems. Taping and de-taping processes can be accomplished in film assisted molding equipment, but issues still arise during block molding high-density leadless leadframes. In a block molding process, a large mold chase is used to form a mold cap over an array of leadless devices before singulation, which separates the individual devices in the array. During block molding processes, the leadless devices can be deflected due to the interaction of shear stresses and bending moments that result from clamping the mold as well as thermally induced stresses. Accordingly, mold flash may still occur during the molding process reducing device reliability.
In flip chip on leadframe packages, solder bump connections between the die and the lead fingers are generally formed using a solder reflowing process. The solder resist pads must properly be defined on the leads or the solder bumps may collapse resulting in incomplete under fill or mold compound coverage in the gap between the flip chip and the leadframe. Additionally, solder dispersion on the leads can result in solder bridging, die placement misalignment, or tilting. One approach to prevent flip chips from dislocating or tilting on the leads is to dispose the solder bumps in concavities formed in the leads and die attach paddle. Solder bumps still may collapse if solder resist pads are not precisely defined around the concavities.
Typical methods of defining solder resist pads for flip chip on leadframe semiconductor packages are labor intensive, time consuming, and not cost effective. In one such method, a non-wettable barrier that separates a wettable solder resist pad from a wettable lead surface is formed using a laser ablation process. In another method, a solder bump with a melting point higher than a eutectic solder paste is used to control the standoff height between the die and the leadframe. Solder bumps still can be dislocated on the leads due to excessive wetting of the solder paste on the leads. The use of a gold stud bumping process also has been proposed, however, stud bumping is a serial process that requires an increased amount of time as the number of bumps required increases. Therefore, expensive, high-speed stud bumping equipment is needed to reduce the manufacturing time. Stud bump processes require more precise die placement equipment and are less tolerant of placement errors than self-aligning solder bump processes. Consequently, the gold stud bump process is more expensive than the typical solder bump process.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.